Replace thoas with OTP json #14

Nine call sites use thoas: directly (ldf_srv, etsi103120, etsi103707, ldf_message_controller, ldf_receiver_controller, ldf_li_controller, ldf_www_controller) but thoas is declared nowhere in this project. It resolves only because nova 0.16 still pulls it in transitively, and Nova 1.0 drops it. ldf_www_controller:safe_decode/1 already uses json:decode/1, so finish the migration: thoas:encode(X) -> iolist_to_binary(json:encode(X)), {ok, V} = thoas:decode(B) -> V = json:decode(B) (raises instead of returning {error, _}).

Surfaced by the review of #13.